(1) Field of the Invention
The present invention relates to a reinforcing fiber. More particularly, the present invention relates to a reinforcing fibrous material comprising a surface-treated, molecularly oriented, silane-crosslinked ultra-high-molecular-weight polyethylene fiber, which is excellent in the combination of the adhesion to a matrix and the creep resistance and is capable of prominently improving the strength of a composite material
(2) Description of the Related Art
Fiber-reinforced plastics are excellent in strength and rigidity, and therefore, they are widely used as automobile parts, electric appliance parts, housing materials, industrial materials, small ships, sporting goods, medical materials, civil engineering materials, construction materials and the like. However, since almost all of fibrous reinforcers of these fiber-reinforced plastics are composed of glass fibers, the obtained composite materials are defective in that their weights are much heavier than those of unreinforced plastics. Accordingly, development of a composite material having a light weight and a good mechanical strength is desired.
A filament of a polyolefin such as high-density polyethylene, especially ultra-high-molecular-weight polyethylene, which has been drawn at a very high draw ratio, has a high modulus, a high strength and a light weight, and therefore, this filament is expected as a fibrous reinforcer suitable for reducing the weight of a composite material.
However, the polyolefin is poor in the adhesion to a matrix, that is, a resin or rubber, and the polyolefin, especially polyethylene, is still insufficient in the heat resisting and the creep is easily caused even at a relatively low temperature.
As the means for improving the adhesion, there have been proposed a method in which a polyolefin molded article is subjected to a plasma discharge treatment to improve the adhesion to a matrix (see Japanese Patent Publication No. 794/78 and Japanese Patent Application Laid-Open Specification No. 177032/82) and a method in which a polyolefin molded article is subjected to a corona discharge treatment to improve the adhesion to a matrix (see Japanese Patent Publication No. 5314/83 and Japanese Patent Application Laid-Open Specification No. 146078/85). The reason of the improvement of the adhesion according to these methods is that, as described in Japanese Patent Application Laid-Open Specification No. 177032/82 and Japanese Patent Publication No. 5314/83, many fine convexities and concavities having a size of 0.1 to 4.mu. are formed on the surface of the polyolefin molded article and the adhesiveness of the surface of the molded article is improved by the presence of these fine convexities and concavities. In Japanese Patent Application Laid-Open Specification No. 146078/85, it is taught that even if the corona discharge treatment is carried out so weakly that the total irradiation quantity is 0.05 to 3.0 Watt.multidot.min/m.sup.2, a very fine haze should be formed on the filament by the discharge, and in Table 1 on page 3 of this specification, it is shown that if the corona discharge treatment is conducted once at such a small irradiation quantity as 0.2 Watt.multidot.min/m.sup.2, the tensile strength is reduced to 60 to 70% of the strength of the untreated filament. It is construed that this reduction of the strength is probably due to the fine convexities and concavities formed on the entire surface.
The improvement of the adhesiveness of the polyolefin fiber as attained in the prior art is due to the increase of the bonding specific surface area or the production of the anchoring effect by formation of fine convexities and concavities on the fiber surface, but reduction of the mechanical strength of the fiber per se by this treatment cannot be avoided. Therefore, the composite material comprising this fiber as the reinforcer is still insufficient in mechanical properties such as the flexural strength.